1. Field of the Invention
The present invention relates to an output circuit which outputs a signal, and concerns control of driving power of an output buffer.
2. Description of the Related Art
In an integrated circuit, a plurality of loads may be connected to one terminal. In this case, if driving power varies from one load to another, it is necessary to output the driving power of a maximum level to all the loads to ensure normal operations for the loads. This means that a load requiring relatively small driving power will be driven with the driving power of an unnecessarily high level, in which case at the time when the load is driven, distortion and radiated noise will occur in a waveform of a signal being output.
There are various techniques for driving a load connected to a terminal in an integrated circuit. For example, Japanese Patent Application Laid-Open No. 2000-295088 discloses a technique where, for each of a plurality of terminals, a current output logical level is compared with a next output logical level, and if the number of terminals for which the logical level changes is greater than a predetermined number, the driving capability of the output buffer circuit is lowered. Further, Japanese Patent Application Laid-Open No. 2003-258611 discloses a technique where the buffer driving capability is changed stepwise along with the changes in the output level to slow the rising and falling of the waveform, to thereby reduce the noise. Furthermore, Japanese Patent Application Laid-Open No. H09-135159 discloses a technique where, at the time of initial settings, the buffer driving capability is set according to an external device being connected.
With the configuration having a plurality of loads connected to one terminal, however, it is difficult to suppress occurrence of the distortion of the waveform and the radiated noise in the waveform of the output signal.